Method for producing improved silicon carbide resistance elements

ABSTRACT

Silicon carbide shapes of the general type shown and described in U.S. Pat. No. 4,125,756 are densified and nitrided in such a way as to produce shapes with improved durability and reduction in temperature span in response to voltage changes.

BACKGROUND OF THE INVENTION

This invention relates to an improved silicon carbide shape of thegeneral type shown and described in U.S. Pat. No. 4,125,756. The shapesproduced by the method described in that patent are densified by dippingthem in furfural, subjecting the furfural coated shapes to fumes ofhydrogen chloride, coated with silicon, and heated. It has been knownfor a long time to apply coatings of silicon carbide to shapes of carbone.g. graphite, or of carburized or otherwise preliminarily coatedrefractory metals such as tungsten, by exposing the shapes to gascontaining a halogenated silane such as methyltrichlorosilane. The priorart methods were suitable for deposition of silicon carbide on densesubstrates, but not upon the porous, incompletely bonded granular bodyof the shape of the type described in U.S. Pat. No. 4,125,756. The priorart is represented by U.S. Patents to Wainer, No. 2,690,409, andClendinning No. 3,317,356, and British Pat. No. 955,700. Of thesereferences, only the British patent suggests that silicon carbide mightbe suitable as a substrate. However, it is clear from the description inthe British patent that a transparent, glassy, probably monocrystalinesilicon carbide layer is to be deposited, and to this end, the Britishpatent is emphatically specific to heating the shape itself totemperatures in the range of 1450° to 1600° C., before exposing it tothe treating gas. For the production of resistance elements from theshapes produced by the impingement of a laser beam on a bed ofunconsolidated particles of silicon carbide, this method isunsatisfactory. In accordance with the process of this invention, apolycrystaline, almost mud-like coating is provided, at depositiontemperatures that are relatively low and deposition rates that arerelatively high compared with the deposition temperatures and rates ofthe methods of deposition of monocrystaline material.

One of the objects of this invention is to provide a process ofdensification and subsequent nitriding that produces a more uniform,durable, and temperature stable shape from a partially bonded granularsilicon carbide substrate than those known heretofore.

Other objects will become apparent to those skilled in the art in thelight of the following description and accompanying drawing.

SUMMARY OF THE INVENTION

In accordance with this invention, generally stated, a porous, partiallybonded, granular silicon carbide body of precisely controlled electricalresistance is given a dense coating of polycrystaline silicon carbide bychemical vapor deposition (CVD) in a hot wall furnace at temperaturesbetween about 1000° and 1400° C., followed by a silicon nitride coating.The result of this treatment is to produce a body that is much strongerthan it was before the treatment. The body will form very little quartzwhen operated at 3200° F., which both increases the life of the body inuse and minimizes clean up in the manufacturing process. The body alsois given a narrower range of change in temperature with changes involtage. Furthermore, the nitriding of the preferred process of thisinvention is done in a way that increases the conductivity of thesilicon carbide and makes the flow of current through the body moreuniformly distributed. The nitriding improves the operation of the bodyin either an oxidizing or a reducing atmosphere, reducing itssusceptibility to oxidation and to silicon monoxide evaporation. In thepreferred embodiment, in which an ohmic contact is embedded in thesilicon carbide body, the silicon nitride is formed in such a way as notto interfere with that contact. The silicon nitride in the preferredmethod is laid down with the thickest protective coating where it ismost needed, in the hottest portion of the body. The preferred processreduces the intensity of any hot spots in the body, to provide a moreuniform heating of the body, which also helps to increase the life ofthe body. Lastly, the surface of the body is further sealed by thesilicon nitride, any remaining pores tending to close with siliconnitride growth.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the preferred embodiment of this invention, a shape, either in theform of an igniter element or larger heating element, is formed by oneof the methods described in U.S. Pat. Nos. 3,764,776 and 4,125,756,preferably by directing onto the surface of a bed of unconsolidatedparticles primarily of silicon carbide, a laser beam with energysufficient to bond, but not completely to fuse, the particles, to form acoherent element, moving the bed and laser beam relative to one anotherin a progression to form a desired shape with ohmic contacts in two endsof the shape, and removing the rind that is formed from the core of theshape.

As is explained in the latter patent, the resulting product is porousand granular, the silicon carbide grains being only partially bonded, inthe sense that they are not wholly fused. At the same time, evenimmediately after their formation, elements in any run of elementsformed by this method have uniform resistance characteristics, withinplus or minus 5% of one another, and in this sense will be referred toas being of precisely controlled electrical resistance.

Preferably the shapes are now suspended by the ohmic contacts, spacedapart along racks supported by boats, and the boats are placed within afurnace chamber which is then sealed. The furnace chamber is heated toapproximately 1270° C. A current of gas consisting of hydrogen as thecarrier, methyltrichlorosilane (MTS) in the amount of about 0.0025 lbs.per liter of gas, and about 0.1% by weight nitrogen, is introduced tothe furnace chamber. Preferably the gas is preheated to about 700° F.,to help maintain the furnace temperature. In this embodiment, a gas flowrate of about 4 liters per minute per square inch of furnacecross-section is used, but the rate of gas flow, the preheatingtemperature, and to some extent, the amount of MTS in the deposition gasis dependent upon the capacity of the furnace and the mass of the shapesbeing treated. If the flow is too fast or if too much MTS is present,spikes will form. If it is too slow, the deposition time is increased.The preheating of the gas is limited by the decomposition of the MTS,premature decomposition of which leads to deposition on the walls of thepreheating chamber.

The inclusion of some nitrogen in the chemical vapor deposition (CVD)gas has been found to lower the room temperature resistance of thedevice, apparently by trapping atomic nitrogen in layers of siliconcarbide. By adding the nitrogen, any other impurity from the furnacechamber and gases is swamped out, so that the doping of the depositedsilicon carbide is made more uniform than it would be otherwise.

Preferably, the boats by which the shapes are carried in the furnace areconnected to docking probes on push rods that project from the furnacechamber. The push rods are reciprocated during the treatment, so thatthe shapes are moved, to break up any static flow pattern of the gas.The gas itself is introduced at high velocity through an orifice, fromwhich it expands to hit the top of the furnace and "explode" across theparts. In this embodiment, igniter shapes (approximately 21/4" long,including ohmic contacts, 1" wide and 1/4" thick at the terminal end and1/8" thick in the serpentine hot zone) are mounted in groups of five,one inch apart, suspended with the plane of their upper surfacesperpendicular to the long axis of the furnace chamber, which iselongated. The boats are reciprocated through two or three inches.However, the spacing of the elements and the amount of movement can andwill be varied with the size and configuration of the furnace chamberand the types of shapes being treated.

For the igniter shapes described, which weigh approximately 1.4 gramsbefore the CVD treatment, a dwell time of 30 to 45 minutes will producea weight gain of approximately 1.2 grams. Because the substrate shapesare so porous, and because the CVD deposition under the conditionsdescribed penetrates the shape a substantial distance, the desiredamount of deposition is generally determined by weight gain rather thancoating thickness.

When the desired amount of silicon carbide has been deposited, theshapes are removed from the furnace and, in the preferred embodimentdescribed, in which the shapes are provided with embedded metalterminals, the terminal regions of the bodies are sprayed with aluminumor other suitable metal to ensure that ohmic contact between the metalterminal and the silicon carbide is maintained throughout the life ofthe element. Only the part of the body immediately around the embeddedmetal terminals is spray metallized, although the entire body isdensified.

Following the spray metallization, temporary electrical connections aremade to the two metal terminals on the element. A cover is placed overthe element and pure nitrogen is flushed into the cover. Thus theelement is surrounded only by nitrogen. Electrical power is applied tothe two terminals until the body reaches about 3300° F. in its hottestregion. In the case of the igniters, this temperature is maintained forabout two minutes. In production, the wattage of the body is monitoredrather than its temperature, determined by a calibration curve ofwattage versus temperature for a given design for the particularfixture.

Nitriding takes place very rapidly within the silicon carbide of theelement. In practice, the element produced and treated by the CVDprocess described has some excess silicon, which forms silicon nitrideon the outermost surface. The silicon nitride forms a denser layer onthe hottest parts of the element than it does on the less hot parts. Inaddition, the nitrogen diffuses into the body beyond the silicon nitrideskin and effectively dopes the silicon carbide, forming a layer oflow-resistance silicon carbide just beneath the silicon nitride skin.Because the doping and silicon nitride formation are functions of thetemperature of the element, the process produces a self-equalizingeffect upon the current flow, and consequently the temperaturesthroughout the element. Thus, if a hot spot exists during the nitridingprocess, nitrogen will diffuse into that area faster and deeper, whichwill in turn decrease the resistance and tend to eliminate the hot spot.Again, the length of time during which the element is exposed tonitrogen will depend upon the character and dimensions of the element.In general, the silicon nitride formation and nitrogen diffusion takeplace in two to five minutes, as evidenced by a pronounced increase incurrent if a fixed voltage is applied, during the first two to fiveminutes, followed by a sharp decrease in the rate of increase. Thisphenomenon demonstrates the effect of the nitrogen doping beneath thesilicon nitride skin, because silicon nitride itself is an electricalinsulator, and its deposition would logically result in a decrease incurrent at a consant voltage, rather than an increase.

When the element is removed from the nitriding fixture, the parts of thebody which were hottest have a different color from the cooler parts andthe surface texture in those areas is also different. No unusual growthis present and the element looks "clean." If the element is operated inair after nitriding, oxygen attack is minimal. In the absence of thenitriding, the body will grow quartz rapidly at 3200° F. The quartzballoons from the body and stretches across the serpentine reaches ofthe igniter. Although the quartz does not electrically short outportions of the body, the element looks bad, and the quartz growthindicates that silicon and carbon have been removed from portions of thebody, which makes those portions liable to develop hot spots. Thenitriding not only improves performance of the element in an oxidizingatmosphere, but in slightly reducing atmosphere as well. In such anatmosphere, silicon monoxide is formed in an untreated element. Siliconmonoxide evaporates at temperatures above 2200° F. Evaporation ofsilicon monoxide and carbon monoxide from the body increases itsresistance, and the element fails within a relatively short period oftime, i.e., 1000 hours or so. With the silicon nitride film, theformation of silicon monoxide appears to be retarded.

Still another benefit of the nitriding process is the decrease of thetemperature range with changes in voltage. For example, untreatedigniters that would measure 2050° F. at 80 volts and 3050° F. at 132volts, a temperature span of 1000° F. will, when nitrided in accordancewith the process described, measure 2150° F. at 80 volts and 3050° F. at132 volts, a temperature span of 900° F. Because most heaters andigniters must be designed to tolerate large variations in applied linevoltage, this narrowing of the temperature span is important. Inaddition, because the nitriding tends to increase the temperatureachieved at the lower range of voltages, the same temperature can beattained at a lower voltage than that required by the untreated element.

In performing the nitriding process, the elements can be separated intobatches in accordance with their room temperature resistance, and thosewith high resistance can be heated to higher temperatures to adjust theresistance of the nitrided element downwardly to a greater degree thanthe elements with lower resistivity at room temperature.

The process of this invention has been described as applied torelatively small igniter elements. It is applicable to much largerelements, as for example, furnace heating elements 18" long,approximately 7/8" wide and 7/8" deep in their hot section and11/8"×13/4" at their ends, U-shaped in transverse cross-section with awall thickness of between about 1/4" in the hot section and 5/16" at theends. Alternatively to increasing the cross-sectional area of the endsto reduce the temperature at the terminals in the operation of theelements, an additive such as silicon or molybdenum disilicide powder toreduce the resistance can be used in those discrete terminal end areaswhen the porous substrate is formed.

The elements may not have metal contacts, although it is preferable toprovide them. When metal contacts are employed, they must be capable ofresisting attack by the hydrogen, or when halogenated silanes are used,the hydrogen and halogen of the CVD gas at the high temperaturesinvolved. Molybdenum or tantalum contacts are eminently satisfactory.

The temperatures and other conditions described above have been found tobe optimum for the particular elements and equipment involved, but, ashas been indicated, some of the conditions and temperatures will bevaried when different elements and different equipment are involved. Thefurnace temperature must be in the 1000° to 1400° C. range. Operatingthe furnace toward the upper end of the range improves (narrows) thetemperature range of an igniter element, but temperatures above 1400° C.do not produce the desired polycrystaline structure. The preheating ofthe CVD gas is not as critical, except for the decomposition of the MTSor its effect upon the equipment if the temperatures are raised muchabove 800° C., and except for the cooling effect on the furnace andelements being treated if the temperature of the gas is too low. In thenitriding process, the temperature of the element must be high enough toproduce atomic nitrogen and low enough to avoid subliming siliconnitride as fast as it is deposited, and to this end, the range should bein the order of 3200° to 3500° F.

The amount of MTS in the CVD gas can be varied, but it has been observedthat the more MTS, the more carbon rich the treated elements become,which tends to produce instability in the elements because the carbonburns out. Too little MTS produces an inferior part because the siliconcarbide is not formed properly. The exact amount of MTS optimum for aparticular type of element, furnace, gas flow, and set of temperatures,can readily be determined in the light of the example given. The amountof nitrogen in the CVD gas can be varied through a considerable range,for example, 0.01 and 3% by weight of the gas. The use of 0.1% providessufficient nitrogen to flush out other impurities. The use of more than3% begins to interfere with the quality of the coating.

Numerous variations in the process and the resulting products of thisinvention, within the scope of the appended claims, will occur to thoseskilled in the art in the light of the foregoing disclosure. Merely byway of example, other materials besides MTS may be used as the source ofsilicon and carbon. Alternative sources are listed in U.S. Pat. No.3,317,356, column 5, lines 37 et seq. Nitrogen, either as molecularnitrogen or in the form of gaseous ammonia or the like can be introducedinto the deposition furnace at the end of the cycle, or silicon nitridecan be vacuum sputtered onto the element. However, the resultingnitriding in either of these approaches is far inferior to the preferredembodiment described. No hot spot correction results in either of them,and in the former, effective spray metallization of the terminal ends ismade difficult if not impossible, because, as has been pointed out,silicon nitride is an electrical insulator, and the spray metallizationwill not ensure ohmic contact between the conductive part of the elementand the terminal. Other methods of forming porous substrates, some ofwhich are disclosed in U.S. Pat. Nos. 4,124,756 and 3,764,776, can beused. Dopants different from nitrogen can be introduced in the CVD gas.These are merely illustrative.

We claim:
 1. The process of forming a silicon carbide resistance elementby chemical vapor deposition comprising forming a porous, partiallybonded granular silicon carbide shape of precisely controlled electricalresistance; placing said shape in a furnace; heating said shape to atemperature of between 1000° and 1400° C.; exposing said heated shape toa gas containing a reducing agent, carbon, silicon and between about0.01 and 3.0%, by weight of the gas, nitrogen to penetrate the shape andproduce a dense coating of silicon carbide containing co-depositednitrogen.
 2. The process of claim 1 wherein the reducing agent ishydrogen and the carbon and silicon are introduced to the gas in theform of a gaseous chloro silane.
 3. The process of claim 1 wherein thegas is preheated to a temperature in the range of 300°-800° C.
 4. Theprocess of claim 2 wherein the chloro silane is methyl trichloro silaneand it comprises on the order of 0.0025 lbs/liter of the gas.
 5. Theprocess of claim 1 wherein a plurality of shapes are placed in thefurnace for concurrent treatment, and said shapes are moved within thefurnace during their exposure to said gas.
 6. The process of claim 1wherein the shapes are two-ended and are formed with metal contactsembedded in the two ends, and the area of the shape around said contactsis spray metallized after the CVD coating process is complete.
 7. Theprocess of claim 1 plus the additional steps of thereafter heating theshape to a temperature above 1600° C. and while so heated, exposing saidshape to gaseous nitrogen.
 8. The process of claim 6 including passingcurrent through the said metal contacts, hence the shape, to heat theshape to above 1600° C., and while said shape is so heated, exposingsaid shape to gaseous nitrogen.
 9. The process of claim 6 wherein themetal contacts are molybdenum.